MEMS (Micro-electromechanical System) microphones are manufactured based on the MEMS technology. In a MEMS microphone, a vibrating diaphragm and a back plate are important components which constitute a capacitor and are integrated on a silicon wafer, so as to realize acoustic-electric conversion.
A package structure of the MEMS microphone is shown in FIG. 1. A MEMS chip 3 and an ASIC (Application Specific Integrated Circuit) chip 2 are surface-mounted on a package substrate 1, and connected via wire bonding, and then a package housing 4 with a sound hole 40 is surface-mounted on the package substrate 1 to form a front cavity of the MEMS microphone. The MEMS chip 3 comprises a substrate 33 as well as a back plate 32, a vibrating diaphragm 30 and the like which are arranged on the substrate 33; and the back plate 32 and the vibrating diaphragm 30 form a capacitor structure for acoustic-electric conversion. The vibrating diaphragm 30, the substrate 33 and the package substrate 1 together form a back cavity of the MEMS microphone. In order to ensure air pressure balance between a front cavity and a back cavity of the MEMS microphone, a plurality of air guide holes 31 is formed in the vibrating diaphragm 30 to realize smooth flow of air between the front and back cavities.
FIG. 2 shows a transmitting path of sound waves in the MEMS microphone. First, incident sound wave enter into a front cavity of the MEMS microphone through the sound hole 40 in the package shell and reach the vibrating diaphragm of the MEMS microphone, so that the vibrating diaphragm thereof vibrates up and down, thereby realizing detection of the sound wave. Most of the sound wave that directly reach the vibrating diaphragm of the MEMS microphone are configured to cause the vibrating diaphragm to vibrate, but a small part of the direct sound wave will pass through the air guide holes in the vibrating diaphragm of the MEMS microphone to enter into the back cavity. As the package substrate is rigid, the sound wave will be reflected and act on the vibrating diaphragm again. The direction of vibrating diaphragm displacement caused by these reflected sound waves is opposite to that of vibrating diaphragm displacement caused by the direct sound waves, offsetting partial displacement caused by the direct sound wave, and reducing sensitivity of the vibrating diaphragm of the MEMS microphone. Moreover, there is a time difference between the direct sound wave acting on a front surface of the vibrating diaphragm and the reflected sound wave acting on a back surface of the vibrating diaphragm. That is, phases are different, which is the same as noise, affecting a signal-to-noise ratio of an output signal.
Therefore, there is a demand in the art that a new solution for a package structure of a MEMS (Micro-electromechanical System) microphone shall be proposed to address at least one of the problems in the prior art.